Under the circumstances that the high speed network society has come, in the mobile media rapidly becoming popular such as mobile phones and laptop computers, development of nonvolatile memories has been particularly required. The nonvolatile memory can retain data without being always supplied with electric power. Therefore, in the devices using the nonvolatile memory can be run immediately after the power is turned on. Further, power consumption can be reduced.
A Magnetic Random Access Memory (MRAM) recently noted comprises high speed characteristics of a SRAM (Static RAM), high density and lower cost characteristics of a DRAM (Dynamic RAM), and nonvolatile characteristics of a flash memory. Therefore, the MRAM is considered as a promising memory of a future de facto standard. The MRAM is a memory using magnetic effects. Of the MRAM, a spin valve type memory utilizing giant magnetoresistive effects, and a memory utilizing spin dependent type tunneling effects are known. In these MRAMs, a switching current is applied to a wiring corresponding to a targeted memory cell and a magnetic field is generated, by which a magnetization state of a recording layer in the cell is changed and bit information is written. Information is read by detecting a magnetization state of the cell by utilizing magnetic effects. As above, the MRAM is a solid-state memory. Therefore, there is no risk of damage, which might occur in a magnetic recording medium for performing writing and reading mechanically by using a magnetic head. The MRAM also excels at repeating writing and reading.
However, for practical use of the MRAM, problems caused along with high density of memories have been left. A magnetic strength required for writing is in inverse proportion to a width of a recording layer, that is, a cell size. Therefore, when a memory cell is miniaturized, a consumption power becomes very large. Further, there is a risk that cross talk may be caused by a proximate leakage magnetic field between adjacent cells. For example, for a memory cell being 0.2 μm wide, a current in writing becomes several mA. Further, for a memory whose cell distance narrows down to about 0.1 μm, when a magnetic field is induced for a targeted cell, a magnetic field having 80% intensity thereof is applied to its adjacent cell.
As a technique to solve the foregoing problems, a MRAM using a new writing method called polarized spin injection method to a recording layer has been suggested (refer to Japanese Unexamined Patent Application Publication No. H11-120758). This memory element is constructed as in FIG. 22. That is, a ferromagnetic layer (fixed layer) 111 wherein a magnetization direction is always fixed and a ferromagnetic layer (free layer) 112 wherein a magnetization direction is changed according to bit information are separated by a paramagnetic layer 113. Paramagnetic metal layers 114 and 115 are electrode layers for applying a current in the laminating direction to the ferromagnetic layers 111 and 112. In the polarized spin injection method, polarized electrons are injected into the ferromagnetic layers 111 and 112 and a spin angular momentum is conveyed by applying a spin polarized current in the laminating direction. Thereby, in the ferromagnetic layer 112, a magnetic moment is inverted by interaction. This mechanism is called spin conversion. In the writing method wherein magnetization is switched by injecting a spin current as above, there is no need to apply an external magnetic field. Therefore, it is free from interference between memory cells, and power consumption can be restrained. The polarized spin injection method is further characterized in that its writing time depends on only a spin conduction rate. Therefore, a response rate can be improved.
However, in this technique, there has been also a problem for practical use. The paramagnetic layer 113 arranged between the ferromagnetic layers 111 and 112 has an aspect as a spin conduction layer for conducting the polarized spin of electrons without relaxation, in addition to a role as a magnetic spacer. Therefore, it is necessary that the paramagnetic layer 113 is made of a material having a long spin coherence length and having a very small spin scatteration to the ferromagnetic layers 111 and 112.
That is, when a spin orientation is changed by, for example, scatteration of spin-polarized conduction electrons in the paramagnetic layer 113, spin information of the conduction electrons have becomes lost. Therefore, the paramagnetic material having a long spin coherence length is desired. To date, researches on spin conduction of the paramagnetic layer have been conducted by using a paramagnetic metal material, a semiconductor material and the like.
However, when the foregoing material is used for the paramagnetic layer, it is difficult to grow a uniform thin film, and to control the spin coherence length. Therefore, there has been a problem that a sufficient spin coherence length and a uniform spin field cannot be obtained in the paramagnetic layer. In the result, regarding the memory element of the spin injection method, though it is theoretically shown that significant effects can be obtained compared to the conventional induced magnetic field method, sufficient characteristics have not been obtained practically. Therefore, its practical use has not been attained.
In view of the foregoing, it is an object of the invention to provide a memory element capable of obtaining a sufficient spin coherence length and a uniform spin field in the paramagnetic layer, and thereby attaining practical use thereof, and a memory device using it.